Generally, an image sensor is a semiconductor device that converts an optic image to an electrical signal. An image sensor is usually an array of charge coupled devices (CCD) or complementary metal-oxide-semiconductor (CMOS) image sensors. In CCD sensors, individual metal-oxide-semiconductor (MOS) capacitors are positioned very close to one another, and charge carriers to be transferred are stored in each of the capacitors. In the CMOS image sensor, the same number of MOS transistors as the number of pixels are formed by CMOS technology using a control circuit and a signal processing circuit as peripheral circuits, and the transistors are switched for sequentially detecting outputs.
CCD sensors have several drawbacks such as their complicated driving methodology, large power consumption, and complicated manufacturing process due to the many mask steps. Furthermore, it is hard to manufacture the CCD as a single chip because a signal processing circuit cannot be embodied in the CCD chip. To overcome the aforementioned problems, research is being directed towards developing the CMOS image sensor using sub-micron CMOS fabrication technology.
In the CMOS image sensor, a photodiode and a MOS transistor are formed in a unit pixel. An image is shown on the CMOS image sensors, each of which sequentially detects signals. Since the CMOS image sensor adopts the CMOS manufacturing technology, power consumption is low. Furthermore, since the number of masks is about 20, the manufacturing process of the CMOS image sensor is very simple, compared to the CCD manufacturing process that needs about 30-40 masks. Furthermore, the CMOS image sensor can be embodied in one chip together with other signal progressing circuits. Due to these reasons, the CMOS image sensor is believed to be the next generation image sensor.
On the other hand, the sensitivity of a CMOS image sensor, one of the most important factors determining its operational characteristics, is defined as the level of signal (voltage) generated by the sensor versus the unit intensity of radiation (lux) being input. The sensitivity is denoted as either mV/lx·s or V/lx·s.
The sensitivity indicates the amount of increase in the output signal (mV) as the intensity of light increases by 1 lux. When the sensitivity is high, the image is more visible even in a dark environment.
Also, higher sensitivity means higher output signal level, which in turn leads to a higher S/N ratio. Two factors determining the sensitivity are optical efficiency and electrical efficiency. A major factor to enhance the optical efficiency is to optimally concentrate external light on a photodiode, that is, to set up appropriate process conditions by operatively associating the thickness and material of an interlayer insulating layer, i.e., the inter metal dielectric (IMD), and the thickness, diameter, material and shape of a micro-lens.
Furthermore, a major method to maximize the electrical efficiency is to optimize a doping profile of a P-N junction so that the light entering the photodiode generates electron-hole pairs to its maximum level at an effective portion. Furthermore, based on the relationship ΔQ=CΔV (ΔQ is increment in charge, C is capacitance, and ΔV is increment in voltage), the capacitance of the capacitor that is used where the signal charges are converted to the signal voltage is sought to be reduced.
The optical efficiency is secured by the process conditions, and the electrical efficiency is determined by the device design.
On the other hand, a general CMOS image sensor simply includes a photodiode, an interlayer insulating layer, a color filter, a micro-lens, and the like.
The photodiode senses light and converts it into an electrical signal. The interlayer insulating layer insulates metal wires from each other. The color filter displays red, green and blue of the light. The micro-lens concentrates the light onto the photodiode.
Among the above, the least-standardized process is the manufacturing process of the micro-lens.
The micro-lens is made of a transparent photoresist material having a gentle oval shape.
According to current semiconductor fabrication technology, a micro-lens in an oval shape is generally manufactured through the exposure and development process using a photoresist and a reflow process.
However, it is hard to standardize the reflow process, and the photoresist used for manufacturing the micro-lens is very sensitive to the reflow process. Accordingly, it is very difficult to improve process stability.
Accurate measurement is thus required for the process standardization. Generally, in a production line, an artificially generated pattern is measured using a critical dimension scanning electron microscopy (CDSEM).
However, in the CMOS image sensor, it is impossible to accurately measure the critical dimension (CD) and thickness of the micro-lens and the color filter.
FIG. 1 is a cross-sectional view illustrating a typical CMOS image sensor.
As illustrated in FIG. 1, the typical CMOS image sensor includes: one or more photodiode regions 31 which generate current according to the intensity of radiation being incident and which are formed on a semiconductor substrate (not shown); an interlayer insulating layer 32 formed on the semiconductor substrate and the photodiode regions 31; a protective layer 33 formed on the interlayer insulating layer 32; a red, green and blue (RGB) color filter layer 34 transmitting light having each specific wavelength and formed on the protective layer 33; a planarization layer 35 formed on the color filter layer 34; and micro-lenses 36 formed on the planarization layer 35, each micro-lens being in a convex shape with a curvature, transmitting through the color filter layer 34, and concentrating light on the photodiode regions 31.
In the typical CMOS image sensor, an optical shielding layer (not shown) is formed in the interlayer insulating layer 32 and prevents the light from being incident on the portions outside the photodiode regions 31.
In the above-described CMOS image sensor, the micro-lens 36 is transparent and has a shape of flat semi-sphere. Each of red, green and blue of the color filter layer 34 is positioned under each micro-lens 36 and planarization layer 35.
The measurement equipment required for the above-described CMOS image sensor shall meet the following characteristics.
First, the measurement equipment shall be capable of measuring the horizontal and vertical dimensions simultaneously.
Since the shapes of the planarization layer 35 and micro-lens 36 formed on the color filter layer 34 vary depending on the shape (the respective thicknesses) of the R, G and B color filter layer 34, simultaneous measurement is required.
Second, the measurement equipment shall be capable of obtaining a large amount of data at once.
Generally, one die includes several millions of micro-lens i.e., one die includes 1,300,000 micro-lens in a 1.3 megapixel sensor). Thus, automatic measurement equipment is needed for a large amount of data.
Third, the measurement shall be performed by a non-destructive method. The measurement equipment shall directly observe and measure the process management of the CMOS image sensor in the production line.
Accordingly, a CDSEM with the above-described characteristics has been used for measuring the process management of the micro-lens and color filter in a conventional CMOS image sensor. The merit and demerit of the CDSEM is that it can obtain only surface information.
Another method for measuring the process management of the micro-lens and color filter in a CMOS image sensor is performed by the measurement equipment using an optic that measures thickness.
In the equipment for measuring thickness, a light source focusing diameter is usually very large. Thus, such equipment is not suitable for measuring a small pattern, unlike the CDSEM.
The cross-section scanning electron microscopy (X-SEM), which is the most common methodology to measure the vertical dimension, uses a destructive method. Thus, it is not suitable for continuous process management of the micro-lens and color filter.